LATE CRETACEOUS BIVALVIA (MOLLUSCA) FROM THE MARÍLIA FORMATION AT THE NORTHERN BORDER OF THE BAURÚ BASIN

MARTÍNEZ, S.; GIARETTA, A.A.; GALLEGO, O.F.

Uberlandia

Jornada; Paleominas 2016; 2016

Universidade Federal de Uberlandia

The Bauru Basin (K) is in southeastern South America and was mostly filled with siliciclastic psammitic sediments. During deposition, the climate varied from desertic to semiarid depending on the time/location. At north, outcrops of the Adamantina and the overlaying Marília formations are found, which are estimated to be of Campanian-Maastrichtian ages. Given the occurrence of conglomeratic strata resulting from alluvial fans, lakes, and braided rivers, the northern and northeastern borders appear to provide records of more proximal deposits. The Echaporã Member of the Marília Formation can also be interpreted as built of moderately developed, well-drained, medium/fine-grained sandstone palaeosols (long sedimentation time lags and wetter periods) with scattered channels. The animal fossil assemblage of both formations includes Spinicaudata, Ostracoda, Gastropoda, Bivalvia, Actinopterigii, Anura, Chelonia, Squamata, Crocodylomorpha and Dinosauria. Herein we introduce specimens of Bivalvia recently found (AAG) in the Marília Formation (putatively Echaporã Mb) from Ituiutaba (MG). At the moment, our sample identification to lower taxonomic levels is not possible because the few freed shells do not show relevant morphological features to systematic. The specimens were recovered from a hill with several levels of roughly graded conglomerate (palaeochannels). Shells were encrusted in ca. 5x5 cm weathered limestone pebbles (bioclasts), many of them rich in 1-10 mm long mechanically-sorted disarticulated valves. The level (or levels) from which they were released still could not be determined. About ten species of bivalves have been recognized for the Bauru Basin, but their putative endemism and rarity still preclude their use in biostratigraphic studies. Highly relevant is the occurrence of the present association as bioclasts, a situation previously recognized to one Jurassic Argentinean unit. The small size and sphericity of the bioclasts suggest lengthy water transport and an allochthonous origin. A recent study envisions the Marília Formation as a plain aeolian area under a dry climate with scattered seasonal rivers. If our fossil bivalves are intra-formational in origin, this palaeoenvironment model should admit the occurrence of long-term (months/years?) water bodies in this system, which could be coincident with the times of low sand deposition and paleosols formation. [* Pesquisador CNPq, financial support by CNPq and FAPEMIG]